This invention relates in general to a transfer apparatus for use for example in an electrostatographic copier, and more specifically to an improved roller transfer apparatus of particular construction to facilitate release of a sheet at a desired time.
In typical electrostatographic reproduction apparatus (copiers or copier/duplicators), marking particles are attracted to a latent image charge pattern formed on a dielectric support to develop such image on the support. The dielectric support is then brought into contact with a receiver member and an electric field applied to transfer the marking particle developed image to the receiver member from the dielectric support. After transfer, the receiver member bearing the transferred image is transported away from the dielectric support and the image is fixed to the receiver member by heat and/or pressure to form a permanent reproduction thereon.
Application of the electric field to effect marking particle transfer is generally accomplished by ion emission from a corona charger onto the receiver member, or by supporting the receiver member on an electrically biased roller holding the receiver member against the dielectric support. While roller transfer apparatus are inherently more complex than corona transfer apparatus, roller transfer apparatus offer certain advantages. For example, roller transfer apparatus typically require a lower energy budget, and also maintain a more positive (physical) control over the receiver member. This positive control is particularly desirable when a receiver member must be recirculated to have multiple images transferred thereto, such as in making multi-color reproductions.
Positive control over the receiver member on the transfer roller has heretofore been provided by mechanical grippers or vacuum mechanisms. Mechanical grippers, such as shown in U.S. Pat. No. 3,612,667 (issued Oct. 12, 1971, in the name of Langdon et al) are of complex construction. For example, the grippers must be recessible within the periphery of the transfer roller to prevent their contacting the dielectric member and causing damage thereto. Vacuum tacking mechanisms, while of a much more simple construction, may not provide sufficient control over the location of the lead edge of the receiver member for accurate registration of the marking particle images on the dielectric member relative to the receiver member. Any misregistration may result in the reproduction on such receiver member being of unacceptable quality. In order to provide more accurate positioning of a receiver member on the peripheral surface of a transfer roller, an apparatus as shown in U.S. Pat. No. 4,724,458 (issued Feb. 9, 1988, in the names of Roy, et al) has been described. The transfer roller apparatus according to this patent includes a locating and vacuum tacking mechanism of simple construction which is capable of accurately locating a receiver member on the periphery of the roller without damaging the dielectric member of the copier.